Mine is a low-level understanding, but when I think of principles of pinning, I think of entering through and adding to some localized hyper-extention until it affects the whole structure as much as possible; while running into an imoveable object (the ground or something extending from it).
Like others with more experience, I also see pinning as a great conditioning tool. In the past, good pins have corrected musculoskeletal problems, making me much more flexible; with greater range of motion and power delivery. Good pins and throws seem to have a way of showing me where my tension is located, helping my ukemi in general.
My tuppence.
Take care,
Matt

I'd like to add: the more spread out "uke's" structure, the less pressure is needed to control it. If they're very hyper/over-extened, it's easier to use "one finger" to pin. My understanding, though, is that many one-finger pins are actually closer to being palm pins with the index finger extended (using "yonkyo knuckle").
I think part of the problem many of the officers have (I'm only guessing; from what I've seen in video) is that they're trying to force a restraining technique too soon and they rely on brute strength to over-power. They often seem to use some immobilization technique and then try to pick them up with it to move them...which seems hard against someone who is "dead-weighting." I would think it better to roll them over, cuff'em, then lift one arm while another officer lifts the other.
Not that this doesn't happen too...just thinking of the few videos I've seen.